Justin Bieber has been pumping iron, and showed off his rippling biceps in a shirtless selfie montage on Instagram. Bieber has definitely filled out from the past few years, when the lanky Canadian pop star weighed a mere 108 pounds.

Justin, 20, has been lifting weights pretty seriously for the past two years in a bid to bulk up. "I’ve been working out in the gym, getting bigger, trying to get sexy for the ladies," he said on Twitter.

Justin previously trained with "The Biggest Loser" star Dolvett Quince, author of the bestseller, The 3-1-2-1 Diet. Quince stopped working with Bieber when he joined "The Biggest Loser" in 2011.

Bieber's current trainer, Patrick Nilsson said Justin typically trains 40 minutes each day, five days a week, mostly lifting weights and focusing on building up his chest and shoulders.

“I want him to look like Marky Mark,” Nilsson told ABC News. “I feel like this is what he needs to get where he wants to be. He’s definitely leaner and a lot stronger.”

Justin doesn't follow a special diet, and Patrick said the focus is on making sure he gets enough calories, since he sometimes forgets to eat due to his busy schedule.

“It’s more about making sure he eats,” said Nilsson. “He’s the only client I allow to have McDonald’s whenever he wants. But he also likes chicken breast, whitefish, salmon, turkey, steak, rice, mashed and regular potatoes and weird greens like cauliflower.”

As it is, Bieber has been super-athletic all his life, having played hockey growing up in his native Canada. Justin has said in TV interviews that he likes water sports, bowling, going to the beach and playing football and basketball.

Was Arrested for DUI Amid Drug Abuse Reports

This has been a rough year for Bieber, who was arrested in January for drunk driving and resisting arrest. The arrest came shortly after reports surfaced that Bieber had developed a serious addiction to the drugs Xanax and sizzurp.

Sources told TMZ Bieber had been using sizzurp, a potent combination of the narcotic pain reliever codeine and promethazine (an anti-nausea drug) mixed with Sprite and a Jolly Rancher.

Sizzurp, which has become popular among hip-hop circles, can trigger seizures or lead to death. In March 2013, rapper Lil Wayne was rushed to the hospital after suffering sizzurp-induced seizures.

Ironically, Bieber insisted in 2012 that he would never abuse drugs or alcohol because he doesn't want to fall prey to the self-destructive behaviors that have marred the lives of other teen stars.

"[Some people are waiting for me to] mess up because a lot of teen stars do," Justin told Oprah Winfrey. "They make that mistake of getting into drugs or alcohol."

But Justin insists that won't happen to him because he surrounds himself with people he trusts that he can confide in about his problems. "Just remembering what's important [curbs any self-destructive tendencies]," said Bieber. "I have to remember to take time for myself to talk to people about what I'm going through, about what's in my head."

In 2013, Bieber made headlines after becoming the first artist with five No. 1 albums before the age of 19 after releasing Believe Acoustic. Justin, who got his start as a YouTube star, revealed his unbelievable rags-to-riches story in his memoir, Justin Bieber: Just Getting Started.

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Samantha Chang is the co-owner of www.theimproper.com., an arts and entertainment website in New York City. A graduate of the University of Pennsylvania, Samantha enjoys running, cycling and taking photos. Contact her at schang@theImproper.com.